Projection systems are becoming increasingly more common, and there is an ongoing effort to make them smaller and less expensive.
In typical three-color projection systems, a red image, a green image and a blue image are formed separately and are then superimposed. In some typical systems, light from an extended, uniform, white-light source is first collimated, and is then spectrally divided into red, green and blue portions. The first collimation step minimizes the angular range over which the spectral filters need to operate.
In many of these typical projection systems, the uniform source is roughly imaged onto a pixelated panel that attenuates the beam on a pixel-by-pixel basis. There are generally three such pixelated panels, with one each for red, green and blue light. In a common pixelated panel, for each pixel, liquid crystal is used to rotate the plane of polarization of pre-polarized light in response to an electrical signal, and attenuation is achieved by passing the exiting beam through a polarizer. The beams leaving the three pixelated panels are then combined and imaged by a projection lens onto a screen, which is typically external to the projection engine. In these common systems, the illumination system typically refers to the optical path extending between the uniform source and each of the three pixelated panels.